_Steel-plate or Copper-plate printing_, in which the subject is printed from an etching or engraving below the surface of a plate of steel or of copper. _Lithography_, in which the subject is printed from a transferred engraving on the surface of a prepared stone. _Typography_, in which the subject is printed from a combination of movable metal types cast in high relief. _Xylography_, in which the subject is printed from a design engraved on a block of wood in high relief. The distinct nature of the substances in use for printing surfaces by the four methods should be enough to teach us that the methods are entirely different. But the manner in which the letters, designs or figures of each method are put on the respective printing surfaces will show the differences more noticeably. In typographic and xylographic work, the matter to be printed is cast or cut in high relief, or _above_ the surface; in lithographic work, it is put _on_ the smooth surface of the stone, in relief so slight that it is almost level with the surface; in steel and copper-plate, it is cut _below_ the surface which receives the impression. The illustration on the next page shows, but in an exaggerated form, the appearance of a [p019] single line, cut across, or in a vertical direction, when it has been prepared for printing by each of the different methods. It will be seen that the line prepared for printing by the typographic or xylographic method can be inked with facility, and that, when compared with a similar line in lithographic or copper-plate work, it presents but a small surface and a slighter resistance to impression. [Illustration: Typography or Xylography. A. Elevated line; the only part of a typographic or of a xylographic surface which receives the ink and impression. B. The shoulder of the type, or the field of the block; it receives neither ink nor impression.] [Illustration: Lithography. C. Transferred surface line; the only part of the surface which receives ink and repels moisture. D. The surface of the stone, that imbibes moisture and repels greasy ink; it receives the full force of impression in every part.] [Illustration: Copper-plate or Steel-plate. E. The line printed, which is engraved below the surface of the plate, and is filled with ink. F. The smooth face of the plate, which makes no mark on the paper, but which receives the full force of impression.] The process of copper-plate printing begins with heating the plate, and rolling it with ink, until the incised lines have been filled. The face of the plate is then wiped clean, care being taken that the ink in the incised lines is not removed. A moistened sheet of paper is then laid on the plate, and an impression is taken by forcing it under the cylinder of a rolling press. Under this pressure, the paper is forced in the sunken lines filled with ink, and the ink sticks to the paper.